The Thermal Hydraulics of High-Pressure Molten Fuel Ejection
Abstract of the technical paper/presentation presented at:
OECD/NEA/CSNI Specialists Meeting on Transient Thermal Hydraulics in Water Cooled Nuclear Reactors
December 13-17, 2021
Prepared by: Thambiayah Nitheanandan
Canadian Nuclear Safety Commission
Design margin assessments of existing nuclear power plants (NPPs), mainly for beyond- design-basis earthquakes (BDBE), began in the early 1990s, using the EPRI methodology for seismic margin assessments and seismic probabilistic risk assessments. Later design margin assessments included the assessment of the containment’s capacity to withstand pressure loads beyond the design pressure value. After the events of September 11, 2001, NPP assessment was extended to include loading cases, such as commercial aircraft crashes and large-area fires, which had not been initially considered during the design phase. The main purpose was to assess the structural limits for existing NPPs and propose beyond-design-basis acceptance criteria that would allow for the availability of the main safety functions, namely: safely shutting down the reactor, cooling the reactor and maintaining the leak tightness of the containment system.
EPRI provided a methodology for BDBEs (or review level earthquakes) to assess the structural capacity of existing NPPs by using acceptance criteria that are less stringent than those currently in the ASME Code (high confidence of low probability of failure (HCLPF) or conservative deterministic failure margin (CDFM)), but which are still sufficient for maintaining the main safety functions.
Based on this experience, the international safety standards (e.g., the International Atomic Energy Agency and the Western European Nuclear Regulators Association) and the regulatory documents (e.g., the U.S. Nuclear Regulatory Commission and the CNSC) used for the design of new NPPs now require – during the design phase – assessments of limits of safe operation of the reactor, as well as any potential cliff-edge effects for loading cases exceeding the design limits. These standards also require the designer to modify the design if necessary. The Fukushima accident justified and reinforced this approach. In line with this approach, what is called “beyond design basis” for the assessment of currently existing plants is called “design extension conditions (DECs)” and “design extension external events (DEEEs)” (IAEA, WENRA, CNSC) for the design of new builds. The idea is the same: to provide acceptance criteria that, despite relatively heavy plastic deformation and structural damage, will allow for the availability of the same main safety functions of safely shutting down the reactor, cooling the reactor and maintaining the leak tightness of the containment system.
Modern design codes and standards recognize the existence of loading conditions higher than design-basis loading conditions and provide design acceptance criteria for these conditions (RCC-CW 2015).
The containment design specifications for new builds now require consideration of design extension conditions and design extension external events.
The current ASME (American Society of Mechanical Engineers) Section III Division 2 Code for the design of nuclear concrete containment structures is based on allowable stresses and essentially elastic behaviour under the application of primary forces. Essentially elastic behaviour is required for all external loading cases and induced primary stresses, except impactive and impulsive loading for which elastoplastic behaviour is allowed.
There is a need to provide the designer with criteria for the structural integrity and leak tightness of the containment building under loading cases higher than design-basis loading cases. The structural behaviour will not remain elastic but the safety functions will still be fulfilled. Depending on the loading cases (seismic, overpressure, impactive/impulsive), the acceptable structural behaviour and the design margins, in terms of the conservatism of code provisions in the elastic domain and the potential excursion to the plastic domain, will be different. Impactive/impulsive loading will affect only a limited portion of the containment structure. The overpressure load will affect the whole containment building, while a seismic load will lie in between these two cases; that is, the loading will affect the whole structure but the maximum forces and ductility demand may be localized. Therefore, the acceptance criteria, in terms of deflections and strains, should be the most stringent for overpressure loading and the least stringent for impactive/impulsive loading.
This paper discusses a proposal to develop acceptance criteria for design extension conditions and design extension external events, along with the design extension load combinations, and provides recommendations for the development of a non-mandatory appendix to the ASME Section III Division 2 Code.
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